<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        /* 清除默认样式 */
        * {
            margin: 0;
            padding: 0;
        }

        ul,
        ol {
            list-style: none;
        }

        .lunbo {
            position: relative;
            width: 880px;
            height: 550px;
            border: 1px solid #333;
            margin: 20px auto;
        }

        .lunbo .pic li {
            position: absolute;
            left: 0;
            top: 0;
            width: 880px;
            height: 550px;
            display: none;
        }

        .lunbo .pic .current {
            display: block;
        }

        /* 按钮 */
        .lunbo .btn a {
            position: absolute;
            top: 50%;
            width: 40px;
            height: 40px;
            margin-top: -20px;
            background-color: rgba(255, 255, 255, 0.3);
            font: 20px/40px "宋体";
            text-align: center;
            color: #fff;
            text-decoration: none;
            font-weight: bold;
        }

        .lunbo .btn .leftBtn {
            left: 20px;
        }

        .lunbo .btn .rightBtn {
            right: 20px;
        }

        .lunbo .btn a:hover {
            background-color: rgba(255, 255, 255, 0.6);
        }

        /* 下标 */
        .lunbo .sub {
            position: absolute;
            left: 100px;
            bottom: 50px;
            width: 240px;
        }

        .lunbo .sub li {
            float: left;
            width: 20px;
            height: 20px;
            margin-right: 20px;
            background-color: rgba(255, 255, 255, 0.4);
            font: 12px/20px "微软雅黑";
            text-align: center;
            color: #666;
            cursor: pointer;
        }

        .lunbo .sub .current {
            background-color: #fff;
        }
    </style>
</head>

<body>
    <!-- 轮播图大结构 -->
    <div class="lunbo">
        <!-- 图片部分 -->
        <ul class="pic" id="pic">
            <li class="current"><img src="images/lunbo/c1.jpg"></li>
            <li><img src="images/lunbo/c2.jpg"></li>
            <li><img src="images/lunbo/c3.jpg"></li>
            <li><img src="images/lunbo/c4.jpg"></li>
            <li><img src="images/lunbo/c5.jpg"></li>
            <li><img src="images/lunbo/c6.jpg"></li>
        </ul>
        <!-- 按钮 -->
        <div class="btn">
            <a href="javascript:;" class="leftBtn" id="leftBtn">&lt;</a>
            <a href="javascript:;" class="rightBtn" id="rightBtn">&gt;</a>
        </div>
        <!-- 小圆点 -->
        <ol class="sub" id="sub">
            <li class="current">1</li>
            <li>2</li>
            <li>3</li>
            <li>4</li>
            <li>5</li>
            <li>6</li>
        </ol>
    </div>
</body>
<script>
    var lunbo = document.getElementById("lunbo");
    var pic = document.getElementById("pic");
    var images = pic.children;
    var leftBtn = document.getElementById("leftBtn");
    var rightBtn = document.getElementById("rightBtn");

    var sub = document.getElementById("sub");
    var subBtns = sub.children;

    var currentIndex = 0;

    var timer = setInterval(function () {
        rightBtn.click();
    }, 3000);

    rightBtn.onclick = function () {
        setImagesIndex(true);
        changeImage();
    }
    leftBtn.onclick = function () {
        setImagesIndex();
        changeImage();
    }
    for (let i = 0; i < subBtns.length; i++) {
        const element = subBtns[i];
        element.onclick = function () {
            currentIndex = i;
            changeImage();
        }
    }
    function setImagesIndex(toRight) {
        if (toRight) {
            if (currentIndex == (images.length-1)) {
                currentIndex = 0;
            } else {
                currentIndex++;
            }
        } else {
            if (currentIndex == 0) {
                currentIndex = images.length - 1;
            } else {
                currentIndex--;
            }
        }
    }
    function changeImage() {
        for (let i = 0; i < images.length; i++) {
            images[i].className = "";
            subBtns[i].className = "";
        }
        images[currentIndex].className = "current";
        subBtns[currentIndex].className = "current";
    }
</script>

</html>